1. Technical Field
This invention relates to coolable airfoil structures of the type used in industrial gas turbine engines, and more specifically, structure for providing cooling fluid, such as air, to a critical location of the airfoil.
2. Background of the Invention
Gas turbine engines for aircraft have rotor blades that are typically cooled to reduce thermal stresses. Reducing the stresses provides the rotor blade with a satisfactory structural integrity and fatigue life. Very complex cooling designs for the interior of the blade have been developed which employ serpentine passages to provide a flowpath for a cooling fluid, such as air.
Heat transfer features, such as trip strips, for creating flow turbulence are typically used in such applications. The trip strip designs have become complex with variations in trip strip height, continuity and angles to the approaching cooling flow in such passages. These designs focus on the microscopic level of increasing the heat transfer in a very small region of the airfoil although they are generally shown as being applied to the entire airfoil. Examples of such designs are shown in U.S. Pat. No. 5,738,493 entitled xe2x80x9cTurbulator Configuration for Cooling Passages of an Airfoil in a Gas Turbine Enginexe2x80x9d issued to Lee; U.S. Pat. No. 5,695,321 entitled xe2x80x9cTurbine Blade Having Variable Configuration Turbulatorsxe2x80x9d issued to Kercher; and, U.S. Pat. No. 4,514,144 entitled xe2x80x9cAngled Turbulence Promoterxe2x80x9d issued to Lee.
These heat transfer features increase the ability of the airfoil structure to transfer heat to the cooling fluid which is flowed through the airfoil. One measure is the heat transfer effectiveness of the structure which is the ability of a portion of a passage to transfer heat across a reference difference in temperature between the wall bounding the leg to cooling fluid flowed through the leg at a given flow rate and temperature. Heat transfer effectiveness is increased by an increase in trip strip height under a given operative condition or a decrease in pitch between trip strips with an increased loss of pressure driving the flow as the flow passes over the features. One convenient parameter to examine in correlating results is the normalized trip strip height to pitch ratio, that is, the trip strip height divided by pitch and multiplied by 100.
Having developed these particular features for small regions of the airfoil, the problem is to use them in a way which promotes heat transfer but does not unacceptably increase manufacturing cost. One area of interest is airfoils for industrial gas turbine engines where complex designs have not been routinely used because of the less severe operating conditions of the industrial gas turbine engine as compared to the aircraft gas turbine engine.
Accordingly, scientists and engineers working on the direction of applicants assignee have sought to develop overall cooling schemes for airfoils of industrial gas turbine engines that provide acceptable levels of heat transfer effectiveness and manufacturing cost.
This invention is in part predicated on the recognition that an industrial gas turbine engine rotor blade is subjected to maximum heat loads under steady-state operative conditions and has locations for these maximum heat loads whose location remains relatively constant for long periods of time on the airfoil. The location does not change even with circumferential variations in flow path temperature such as might occur with can-type or can-annular combustion chambers. Thus, the time at temperature location is relatively fixed in comparison to aircraft gas turbine engine airfoils. In aircraft gas turbine engines, the maximum heat load typically occurs during transient periods at sea level takeoff condition and decreases in size for the steady-state cruise condition. The size and location of the maximum heat load at these conditions shifts on the airfoil due to different cooling air flows between the conditions and different flow path heat loads on the airfoil because of different temperature and gas velocities in the flowpath. This enables more tailoring of the internal cooling passages for an industrial gas turbine engine and some flexibility in design which permits forming a blade which is more easily fabricated than airfoils for aircraft gas turbine engines. This might occur in industrial engine airfoils by reducing variations, for example, in the designs of the arrays of trip strips as compared to aircraft engine airfoils which must accommodate the shifting locations of maximum heat transfer between operating conditions. In addition, this invention is in part predicated on the recognition that the maximum heat load occurs at the leading edge region and at the trailing edge region for an industrial gas turbine airfoil in one known application which has an airfoil having two serpentine passages which each have three spanwise legs serially connected for flowing cooling air from the midchord region to the leading edge region. The cooling air is flowed to the third leg of each serpentine passage which is closest to an associated edge region of the airfoil and is discharged via the third leg from the serpentine passage. In this known configuration, a leading edge cooling air passage is disposed between the serpentine cooling air passage and the leading edge.
According to the present invention, a coolable rotor blade for an industrial gas turbine engine has an airfoil having two serpentine passages which each have three spanwise legs serially connected for flowing cooling air from the midchord region to an associated edge region of the airfoil and wherein each leg of each serpentine passage has a trip strip height to pitch ratio over at least a portion of the leg which is greater than that of the preceding leg to increase the heat transfer effectiveness of each downstream leg over that of the upstream leg as the cooling air in either serpentine passage moves closer to the associated edge region of the airfoil and has a pitch for the arrays of trip strips in each leg which is constant (except for the rearmost leg which discharges the cooling air along the passage) to promote ease of manufacture and inspectability while providing heat removal that emphasizes the edge regions over the midchord region.
In accordance with one embodiment, the third legs of both the front serpentine passage and the rear serpentine passage have an increased trip strip height as compared to the other legs over at least a portion of the third leg.
According to the present invention, the third legs of both the front serpentine passage and the rear serpentine passage have both a trip strip height and normalized height to pitch ratio over at least a portion of the third leg which is greater than that of the second leg to provide a greater increase of heat transfer effectiveness to the third leg over the second leg than exists between the second leg and the first leg as the flow of cooling air proceeds in the downstream direction.
In accordance with one embodiment, the third leg of each of the two serpentine passages has a first portion which receives cooling air from the second leg and a second portion outwardly of the first portion, the second portion having a trip strip height and a trip strip height to pitch ratio which is greater than that of the first portion and the second leg having either a trip strip height or a trip strip height to pitch ratio which is greater than that of the first leg to sequentially increase the heat transfer effectiveness of the second and third legs in the downstream direction.
In accordance with one embodiment, the first and second legs of each serpentine passage having arrays of trip strips each have no increase over the entire first leg or the entire second leg in the trip strip height and trip strip height to pitch ratio.
In accordance with one embodiment, the rotor blade has a third passage disposed between the leading edge and the third leg of the forward most serpentine passage, the third passage having a first portion adjacent to the root, a second portion adjacent to the tip and having a trip strip height and trip strip height to pitch ratio which is greater than that of the first portion.
A primary feature of the present invention is a coolable airfoil having a front serpentine passage and a rear serpentine passage with the inlet legs disposed in the midchord region having trip strips in the front serpentine passage but not in the rear serpentine passage. Another feature is the height of the trip strips in the legs of each passage which is unchanged for most legs over the length of the leg. Another feature is the pitch of the trip strips in the legs of each passage which is unchanged for most legs over the length of the leg. Still another feature is the variation in the trip strips from leg to leg and the heat transfer effectiveness of the legs which increases sequentially in the downstream direction. Still another feature is the angular orientation of the trip strips which forms an acute angle with the adjacent structure bounding each serpentine passage. Another feature in one embodiment is the angular orientation of the trip strips in the third passage which form an acute angle on the upstream side of the trip strip with the rib of the front serpentine passage.
A primary advantage of the present invention is the cost of the airfoil which results from the ease of inspection and manufacture of the coolable airfoil having relatively constant height and constant pitch trip strips in most legs of the cooling passages for the airfoil. Another advantage is the durability of the airfoil under operative conditions which results from acceptably cooling the leading edge region and the trailing edge region of the airfoil while shifting some of the cooling load from the midchord region to the front serpentine passage and bolstering cooling of the region cooled by the front serpentine passage with a leading edge passage.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of the invention and the accompanying drawings.